This invention relates to an inverter circuit, particularly to improvements in an inverter circuit for driving a AC motor.
It has become possible to control the speed of recently developed AC motors by varying the frequency of an alternating current delivered to the motor, from the output side of an inverter. The type of inverter involved comprises switching elements such as a plurality of transistors or thyristors connected in the form of a bridge, and converts a direct current applied to its input side into an alternating current due to the switching action of the switching elements. In turning on and off, the switching elements in the inverter produce sharp electrical spikes which are a source of noise and which may even damage the elements of the inverter. To prevent such phenomena, a so-called snubber circuit comprising a capacitor, resistor and diode in parallel is connected across both ends of each switching element so that the spikes produced by the switching elements may be absorbed within the snubber circuit. Such an arrangement is disadvantageous because, providing each switching element with a snubber circuit complicates the overall circuitry. An improvement has been to employ circuitry wherein a capacitor of large capacitance is connected to the inverter input circuit for the purpose of absorbing spikes produced by the inverter.
In a case where the improved inverter arrangement is used to drive an AC spindle motor of a machine tool, safety requires that the DC voltage applied to the inverter be cut off when the machine is not in use. To this end, a contactor must be provided on the inverter input side. When the contactor is closed (i.e., switched "on") to use the machine tool, a large momentary current flows into the spike absorbing capacitor from the source of direct current, requiring that the source, such as a rectifying circuit, be large enough in size to withstand the large momentary current. This is a requirement that, because of cost, results in poor economy. Another shortcoming is that each time the machine tool is actuated, a large rush current flows into the spike absorbing capacitor, thereby shortening its life.